A Matter of Degrees

Even at their best, we can never see the full face of Mercury or Venus

By J. Alex Knoll

While winter has just begun, we’re already in the process of reclaiming daylight, and Saturday marks a milestone when the sun sets at 5:00. Over the next month, the sun sets roughly one minute later each day. That same Saturday, daybreak arrives at 7:25, but alas, through January, it will come just a few minutes earlier each week.

Sunset reveals Jupiter in the south. Look for the waxing moon less than 10 degrees to the east of Jupiter Sunday night and to the west Monday night. By 10 both nights, moon and Jupiter hover above the west horizon, and by 11pm they are gone.

An hour later, Saturn rises in the east, climbing higher and arching south through the wee hours of the morning. The ringed planet shines with a steady, golden glow, not to be confused with the dimmer, blue-hued and twinkling light of Spica trailing Saturn by several degrees.

Around 4am, Venus rises in the southeast and easily outshines any other light in the sky. Saturday marks our sister planet’s greatest elongation — its farthest point west of the sun and its highest point — 47 degrees — above our horizon at daybreak.

The next morning marks greatest elongation for our other inner planet, Mercury. Look for it just above the east-southeast horizon an hour before dawn. Shining at zero magnitude, Mercury is brighter than all but a few stars. But it clings so close to the sun that even at greatest elongation it is only 15 degrees above the horizon at sunrise.

Seen through a telescope, both Venus and Mercury appear only partially illuminated. Because the orbit of these two planets lies inside of our own orbit, we can never see them when they are a full 180 degrees opposite the sun from us. In fact, Venus at its greatest elongation is only 47 degrees from the sun, appearing half-lit, and Mercury at best appears less than 24 degrees from the rising sun.